The present invention relates to a method of generating a servo signal for a servo system used to control the position of a data sensing light spot with respect to recording tracks on a recording disc, in a recorded data playback apparatus of the type which employs an optical system (e.g. based on a laser beam) for data read-out from a recording disc, generally referred to as an optical disc player or laser disc player.
In particular, the present invention is related to a method of generating a tracking servo signal for a servo system of the type in which sensing is performed using a single light beam, which is reflected from the disc surface onto a pair of closely mutually adjacent photo-electric elements, to thereby derive a pair of sensing signals having respective low frequency components therein (assuming that the light beam is moved radially across the recording disc at constant velocity with the tracking servo loop opened) which vary in phase in accordance with the deviation of the light spot formed on the recording disc by the light beam from a position at the center of a recording track. Each recording track consists of a series of pits, each being a minute elongated depression in the recording disc surface, of a contour approximating to an arc of a circle, as viewed in cross-section along the track direction. The diameter of the light spot is less than that of the spacing between the centers of a pair of mutually adjacent recording tracks. The level of light reflected from such a disc is a maximum when reflection occurs from the center of a recording track. Thus, if the light spot is moved across the rotating disc surface, along the direction of a radius of the disc, the portion of reflected light falling upon one photo-electric element will reach a maximum level when that portion is located at the center of a recording track. As the light spot is moved further, the portion of reflected light falling upon the other photo-electric element will then reach a maximum, and so on. Thus, if the optical system is perfectly aligned with respect to the disc surface and the photo-electric elements, the output signals from the photo-electric elements (to be precise, the low-frequency components of these output signals, with the RF component due to the modulation of the reflected light by the data recorded on the recording tracks having been removed) will become equal in level when center of the light spot is located at the center of a recording track. This is the basis of the "single beam" type of tracking servo system, since in principle it is only necessary to continuously control the optical system such that the output signals from the two photo-electric elements are identical in level.
It is a feature of such a recorded data playback apparatus that the spacing between adjacent recording tracks is extremely minute, so that the light spot, i.e. the light spot which is focussed onto the recording disc surface, must also be of very small diameter. It is therefore necessary that any tracking deviation, i.e. deviation of the light spot from a position centrally disposed on a recording track as the recording disc rotates, must be held to a very low level. Thus the tracking servo system which controls the optical system, e.g. by displacing the object lens of the optical system in a direction along the disc surface at right angles to the track direction, such as to compensate for any such tracking deviation, must provide an extremely high degree of control accuracy. In prior art methods of deriving and applying a servo signal for such a servo system, the system is adjusted, prior to use, only such that the tracking error signal, i.e. the servo signal contains no DC component, i.e. no DC offset. However the applicant has found that such a method is not completely satisfactory, since optimum elimination of this DC offset is not attained for various different positional relationships between the optical system used for data sensing and the recording disc. That is to say, if, due to manufacturing deviations the recording disc should be slightly tilted with respect to the direction of the light beam which produces the light spot, i.e. with respect to the optical axis of the optical system, or if the object lens (which focusses the light beam onto the recording disc to form the light spot referred to above ) in that optical system should be shifted from a correctly aligned position with respect to the main optical axis of that system, then optimum compensation for tracking deviation cannot be attained by simply adjusting the value of offset voltage of the servo signal to zero when the light spot is centrally positioned on a recording track.
The present invention therefore describes a method of generating such a servo signal whereby optimum compensation for DC offset in the tracking error signal can be obtained, for a recorded data playback apparatus in which a certain amount of inclination of the recording disc or positional deviation of the object lens can be anticipated.